Two Types of Unhappy Online Research Results--None and Too Many

First: distinguish between "search engines" and the databases you tell them to search

Unhappy Result #1: Zero Results--

You are searching the right database but your search uses terms the experts do not use.

OR

You are using the right search vocabulary in the wrong database.

Unhappy Result #2: Too Many Results--

You are searching the right database, but you are using search terms that are not precise enough.